Voltage control system for cathode-ray tubes



Patented June 14, 1949 VOLTAGE CONTROL SYSTEM FOR CATHODE-RAY TUBES Alfred mins... a ca a, a s nr to, Radio Corporation of Arnerica, a corporationof' Delaware r l ation a 28, 1947; S ia N.0- 75 .10.7

4 Claims.

1. he--p ssen nventn relates. to t ode ray b ircuits. and mor nar c rrelates.

p v dme n f r developin pote tials..w i h potential. gr die t. betw en. t ub first second. anodes, As a resultoi their, diiierence in potential, thesetwo elements, act, in effect as an electron lenswhich, causes the electrons which make up. the, scanning, beam. normally to. converge. at a: pointon, the screen or target of the tube. The. point where the, issuing beam will focus. is, determined. in part .by. the configuration of theconstituent, electrodesand also by both the absolute and relative magnitudes of, the potentials applied: thereto,

Cathode .rayv tubes. as utilizedin many presentd y p ications. uch,,f0r example, s in the t vision field, requirekrelatively high potentials onv their accelerating electrodes in order that the image reproduced on the screen of the tube reaches, a desired brightness level. The second anode potential of certain. cathode. my image-. repr d n tubes tilizedin. r t c i nyp e vision receivers, for example, may reach. a value. in. the order of. 30.kiloy olts, or, even higher, relative to, the cathode, 'I-o produce-a, iocussing of the scanning. beam, the. first. anode of the tube. should be operated at.a .relatively lower potential, inorder to. establish. theelectrqstatic field between these two, elements Whichis essential to r n abou a. converg nc of he s annin am on. he. lumines nt stree since. thefirstanoda of the cathode ray tube is normally. operated. at a; potential lower than thatof thesecond anode, his-customary in many circuit arrangements tocouple the output of some suitable n. 0. pow r supp y d ectly to he atter, and then connect. a series, of relatively, highresistor elements (or bleeder ).between. the output of the power supply. and ground. If one of these resistor elements is provided. with an adjustable tap-, then a variable. output voltage is obtainable therefrom which. may be applied to the first anQdQ of the cathode ray tube so as to change the potsntial gradient. between the first andsecond anodes. and hencethe focusing, of. the scanning beam. of the; tube.

In certain cathode ray, tube, applications, however, and especially where. tubes requiring unusually high second. anode. potentials are em.- ployed, the use 0f. s lfih-fliblfifid resistor may not in every. case, be completely satisfactory. Insome cases, the carbon potentiometer, which is. frequently incorporated the. bleeder resistor in order to obtain the. first anode. potential, is subject,to arcing which causes.its eventual breakdown. Wire-Wound. p tenti,ometersI would, eliminate this difficulty, butthey. are not readily available in values. over 20.0,00Qohms. Furthermore, the potentiometer control itselfi operatesat substantially the. potentialof. the cathode raytube first anode, and hence. the, problem of. adequate insulation. is. a, factor. Another disadvantage occasionally encoiu'itered in installations where single-voltage packsof. very high voltage areused (such, for example, as in: large size. screen projection-type television, installations such, as have been. used for theatres, for instance). is that of providing, suitable. corona shields for the control elements. of. several, serieseconnected. potentiometers. This, additional. equipment. adds.- to the expense and. complexity of, the power supply apparatus.

In arrangements. where. a cathode ray tube of the type having asubstantially flatface is em.- ployed,. it occasionally,- difficult to. maintain adequate focussing over the. entire image raster area of the tube, for theiobyious, reason that an increase in the beam. deflection. angle. causes. the beam, to travel over alongerpath beforestriking the screen, This. results, a defocussing of. the cathode ray beam, near the. edges.- of the image raster unless Proper steps, are taken to provide some system of. focus modulatiqn Where, for example, focussing varies as afunntionof. deflection amplitude. Such systems; are now. ktmwn in the art, but.req,uire. additional equipment, including, at leastpne,extraslfiqtron discharge, tube, i order to accomplish thisresult.

Although many occasions, arise in, practice.

when it is. desirable to. adjustthe iocussing. potential. Qt-a cat ederar be ov r a tiv l wide manufacture or cathode. ray tubes. in. order. to. p mit ad qu e test ng he ube charact cs This-is llustrated. r e amr abr a. pr Q clu e f equentlyresqrte ltebr athQ-d r y tub manufacturers. to check the emitting suriace of the cathode. In this procedure, the control grid potential of the tube is varied over a range from zero to about volts, for example, or to a point at which the cathode surface and the grid aperture come into focus. A satisfactory cathode emitting surface will then produce a uniformly bright spot on the tube screen, while a poorly formed, or damaged, cathode surface will contain one or more dark areas. In order that the above procedure be efiective, however, it is necessary that the focussing potential on the first anode of the tube be reduced to a point considerably below its normal operating value, and special equipment may be necessary in the event that the range of the normal focussing potentiometer is inadequate.

Another process frequently employed in the testing of cathode ray tube characteristics makes use of a so-called life-test rack, wherein a number of tubes of different types may be operated for varying periods of time from a common source of anode #2 potential. In this case, it is desirable that the anode #1 potential be adjustable over a wide range in order to permit each of the tubes being tested to be focussed individually.

In accordance with one feature of the present invention, a system is provided for obtaining an adjustable value of high voltage from a fixed voltage source of higher value. These voltages, for example, may comprise the first and second anode potentials, respectively, of a cathode ray imagereproducing tube. This is accomplished, in one embodiment of the invention, by utilizing a gridcontrolled electron discharge, or regulator, tube in series with a voltage dropping resistor. This series combination of regulator tube and resistor is connected between ground and the positive terminal of the fixed high voltage source, or, in other words, between the cathode and second anode of the cathode ray tube. By varying the conduction characteristics of the regulator tube through the control of its grid potential, the current flow through the dropping resistor may be adjusted to any desired value. The voltage appearing between the series-connected regulator tube and resistor will thus be determined by the magnitude of the current flow through the resistor, and, since this current flow may be controlled in the manner above set forth by varying the tube grid potential, it is possible to connect the first anode of the cathode ray tube directly to that end of the dropping resistor on which this desired voltage appears. It will be understood, of course, that this connection may be made, if desired, to a point on the dropping resistor intermediate its ends so as to obtain a voltage drop of lesser magnitude.

In its broader aspect, the invention encompasses the use of any suitable variable-impedance device in place of the grid-controlled regulator tube above described.

In another embodiment of the invention, the bias voltage on the grid of the regulator tube is modulated by a further cyclically varying voltage the instantaneous magnitude of which is a function of the cathode ray beam deflection angle. In this manner, the focus of the cathode ray beam may be constantly changed in accordance with the degree of beam deflection, thereby maintaining correct focus over the entire image raster area, and eliminating the loss of definition near the edges of the raster now experienced in certain systems when no such focus modulation is employed. It will be noted that, in this embodiment of the invention, a single grid-controlled electron discharge, or regulator, tube is utilized for the dual '4 purpose of providing focus modulation and also for controlling the anode #1 potential, thus combining two functions in a single tube.

One object of the present invention, therefore, is to provide an improved circuit for obtaining an adjustable value of high voltage from a fixed voltage source of higher value.

Another object of the invention is to provide an improved system for obtaining a variable potential suitable for application to the first anode of a cathode ray image-reproducing tube.

A further object of the invention is to provide a system for obtaining an adjustable focussing potential for the first anode of a cathode ray tube from a unipotential high-voltage source without the employment of a so-called bleeder resistor.

A still further object of the invention is to provide a system in which a single electron discharge device may serve the dual purpose of regulating the normal focussing potential applied to the first anode of a cathode ray tube and also of permitting the focus of the cathode ray scanning beam to be varied as a function of variations in the beam deflection angle.

Other objects and advantages will be apparent from the following description of a preferred form of th invention and from the drawings, in which:

Fig. 1 is a partially schematic representation of a preferred form of voltage control system in accordance with the present invention; and

Fig. 2 is a modification of Fig. 1, showing the addition of a source of modulating potential for the grid of the regulator tube.

Referring first to Fig. 1 of the drawings, there is shown a cathode ray tube generally indicated by the reference numeral l0. Tube Hl may be of known construction, such, for example, as the image-reproducing tube, or kinescope, employed in television receivers, and hence will not be described in detail. It will be understood, however, to include a cathode I2, a heater element l4, and a control electrode or grid 5 to which video signals from a television receiver (not shown) may be applied over a conductor [8. Of course, where the cathode ray tube I0 is not employed in connection with a television receiving system, any other form of modulating potential may be supplied, if desired, to the control grid 16.

The cathode ray scanning beam developed Within the tube l0 (indicated schematically in the drawing by the broken line 20) is accelerated toward the luminescent screen 22 of the tube by means of two accelerating electrodes 24 and 26, the electrode 24 constituting the first anode of tube It and the electrode 26 comprising the second anode thereof. Tube III has also been illustrated as including a conductive coating 28 formed on the inner surface of the flanged portion of the tube, this coating 28 extending into the neck of the tube for a sufiicient distance so that it may be electrically connected to the second anode 26 by means such as the spring contact 30. However, this particular type of cathode ray tube construction has been illustrated merely as an example, and it will be appreciated that the conductive coating 28 may be omitted, for instance, and the second anode 26 connected directly to a source of operating potential of the type to be hereinafter described.

Tube Ill is also provided with a deflecting yoke which may include a set of line deflection coils. for deflecting the cathode ray scanning beam 2H in such a manner as to scan each line of the image raster produced on the target or screen 22, and also a pair of field deflection coils for amete deflecting the beam 20 in a-direction substantially perpendicular to its line-scanning .direction. Since the deflecting yoke 30 may be of any suitable known construction, a description of its details will be omitted. It will in addition be understood that both line and field deflectin potentials are supplied to the yoke 30- from defiection generators having current outputs of suitable waveform. Since the means for deflecting the cathode ray beam 20 forms no part of the present invention, however, the deflecting yoke'3ll maybe replaced by two sets of deflecting plates within tube 18, thereby providing for electrostatic deflection of the beam instead of causing it to be deflected by theelectroma'gnetic apparatus illustrated.

In order that the cathode ray scanning beam 20 of tube Ml be properly focussedon the screen 22 so as to produce an image of high definition, a potential must be applied to the first anode 24' which is of a value different from that of the potential applied to the second anode 28. This difference in potential between theelectrodes 24 and 28 produces an electrostatic field which acts in effect as an electron lens to cause the electrons in the beam 28 to converge. The distance of this point of convergence from the anode 25 is determined in part by the relative "magnitude of the two potentials. For an extended discussion of the principles underlying the action of the electrostatic field formed by the electrodes 24 and 26, reference is made to the book Electron Optics in Television, by I. 'G. Maloff and D. W. Epstein, published by the McGraw-Hill Book Company in 1938.

The conductive coating 2.8 on the inner surface of tube It) is joined by means oia conductor 3! to the positive terminal 32 of a high-voltage D.-C. source applied betweenthe terminals '32 and 34. The negative terminal '34 of this highvoltage D.-C. source is grounded, as shown, and

is also connected to the cathode [2 of tube II].

In accordance with the present invention, the potential applied to the first anode 24 is obtained from a circuit which includes an electron discharge, or regulator, tube 38 and a dropping resistor 48, these elements 38 and 40 being connected in series across the terminals 32 and 34 of the high-voltage D.-C. source which supplies potential to the second anode 26. Tube 38 includes at least an anode 42, a cathode 44, and a control grid 46. The anode 42 of tube 38 is connected to the opposite end of dropping resistor 4'8 from that connected to the terminal 32. It is also connected to the first anode 24 of cathode ray tube Ill by means of a conductor 48. Cathode 44 of tube 38 is grounded. The control grid 46 of tube 38 is supplied with an adjustable bias, or control voltage, from a tap 50 on a potentiometer 52, the latter being connected across a relatively low-voltage source 54. The lowvoltage source 54 may be of any type having a maximum rating of 700 volts, for example, while the high-voltage D.-C. source applied between the terminals 32 and 34 may have a value of 30 kv., or even higher.

The positive terminal of the low-voltage source 54 is connected to the cathode 44 of tube 38. Thus, by varying the position of the tap 50 of potentiometer 52, an adjustable bias voltage may be applied to the grid 4'6. The value of the voltage source 54 is so chosen, in accordance with the operating characteristics of tube 38, that by varying the position of the adjustable tap 50, the

current flow through tube 38 may be reduced. to cut-off of the tube.

It will now be seen that when tube 38 is cutoff, its anode 42 is at substantally the samep'otentia'i as the positive terminal 32 of the high-voltage D.-C. source supplying energy to the second anode 26. In other words, the first anode 2 4 and the second anode 28 of tube H] are at very nearlythe same potential (neglecting any slight voltage drop which may be produced across resistor-40 by a flow of first anode current). However. when tap 50 is adjusted so that tube 38 conducts, current flows through resistor 40 to produce a voltage drop across the resistor. The potential on the anode 42 of tube 38 is thus decreased relativeto the potential at terminal 32. As the tap 50 is ad justed so that a greater current flows through tube 38, the voltage on the first anode 24 of tube ID will decrease until a point is reached wherethe current flow through tube 38 is at a maximum. The anode of tube 33 will then be substantially at zero, or ground, potential.

It has been found in practice that if the regulator tube 38 is chosen to be of type 808, and if resistor 48 is chosen to have a value in the order of 24 megohms, then a variation in the bias potential applied to grid 48 of tube 38 from 3-50 volts to 75 volts will cause the potential applied to the first anode 24 of tube ID to vary from approximately 12 kilovolts (the voltage at terminal 32) to zero volts.

Since regulator tube 38 provides a relatively low-impedance path between the first anode 24' of tube It] and ground during the time that this tube is conductive, it will be seen that failure of tube 38 for any reason during operation of the circuit will suddenly throw the entire voltage at point 32 onto the first anode 24, resulting in possibility of damage to one or more portions of the cathode ray tube. In order to provide for this contingency, a protective gap 58 may be connected across tube 38 in the manner shown in Fig. 2 of the drawings. The dimensions of this gap 58 are preferably so chosen that the presence thereacross of a voltage in excess of the maximum safe value of anode #1 potential will cause the gap to are over and thus prevent damage to the cathode ray tube iii. If desired, this gap 58 may be omitted and a resistor (not shown) of relatively high value substituted therefor. In fact, it may in some cases be desirable to employ such a resistor in shunt with the tube 38 to not only act as a protector for the cathode ray tube It but also to modify the controlling action of the tube 38.

Since the fiow of current through regulator tube 38 is controlled by the value of the bias applied to its grid 48, it is possible to vary this grid bias in such a manner that the voltage on the anode 42 varies in a predetermined cyclic manner. One method of accomplishing this result is to connect the grid 48 of tube 38 to a source of modulating potential 58, which may be of any desired type and which has, therefore, been shown schematically in Figure 2. The output of the source 58 may be a modulating voltage of any selected waveform. For example, the voltage output of source 58 may vary as a function of the fiow of current through the deflecting yoke 30 in Fig. 1, thereby providing for focus modulation of the beam 20 in the manner previously set forth. It will also be obvious that many other types of modulating potentials may be applied to the grid 46 of tube 38, depending upon the particular results desired.

I claim:

1. In a system for supplying operating potentials to two electrodes of a cathode ray tube in order to effect focusing of the scanning beam in said tube, the combination of a source of high D.-C. voltage, a circuit for applying said voltage to one of said electrodes, a resistance, a grid-com trolled electron discharge device, means connecting said resistance and said device in series across said source of high D.-C. voltage, an independent source of relatively low bias voltage, a circuit for applying said bias voltage to the grid of said electron discharge device to limit the flow of current through said device and also through said seriesconnected resistance, a connection from another of said electrodes to a point on said resistance, a source of modulating potential, and means for applying said modulating potential to the grid of said electron discharge device to modify the action of said bias voltage.

2. In a system including a cathode ray tube of the type in which two electrodes in said tube are arranged. to form in effect an electron lens which acts to bring to a focus the electrons forming the scanning beam in said tube when operating potentials of different values are applied to said electrodes, and wherein the focal length of the electron lens so formed is dependent in part on the relative values of the said operating potentials, the combination of a voltage source of fixed value, means connecting the positive terminal of said voltage source to one of the two electrodes, a resistor having one end thereof connected to said positive terminal, a variable-impedance device connected between the other end of said resistor and the negative terminal of said voltage source, a comiection between a point on said resistor and the other of the two electrodes in said cathode ray tube, a cyclic potential corresponding to the amplitude of the deflecting current for said scanning beam, and means for varying the impedance of said device by said cyclic potential.

3. In a circuit including a cathode ray tube having two accelerating electrodes and a source of smooth D.-C. potential connected directly to one of said electrodes, the combination of resistance and adjustable impedance connected in series across said smooth potential source, a connection from another of said electrodes to the common point of said resistance and impedance whereby an adjustment of said impedance results in adjustment of the smooth potential applied to the last-mentioned electrode, a source of potential which varies as a function of the defleeting current of said cathode ray tube, and means for modulating by said varying potential the smooth potential applied to said last-mentioned electrode.

4. A voltage control circuit for a cathode ray tube having first and second anodes, comprising means for supplying a unidirectional voltage to said second anode, a dropping resistance connecting said means to said first anode, means comprising a variable impedance device connecting said first anode to a point of fixed potential, said device having an element for controlling the impedance thereof, and a source of potential responsive to the deflecting current of said cathode ray tube, said source being connected to said control element for modulating the voltage on said first anode according to the amplitude of the defleeting current.

ALFRED W. COMINS.

REFERENCES CITED The following references are of record in the die of this patent:

UNITED STATES PATENTS Number Name Date 2,276,455 Beers Mar. 17, 1942 2,434,196 Cawein Jan. 6, 1948 

